There’s no such thing being a fail-proof custom thermocouples. Over time, thermocouples fail. To compensate for this, a temperature controller will probably go into upscale burnout mode, and drive the furnace burner to low fire or reject the SCRs. But then, you suffer from the downtime, rework, or perhaps the potential of losing product.
Not long ago, a plant operator called to determine if there we had ways to work around this burnout mode, so he wasn’t wasting time and materials.
His heat treat load had almost finished its final soak once the control thermocouple broke open. The controller, as expected, drove the furnace burner to low fire. The operator then popped the controller into manual mode, so he could nurse the load throughout the remainder from the soak cycle. He used the temperature reading over a recorder, fed from the second, unbroken thermocouple inside the protection tube as temperature indication to the load.
In case the situation had happened in the midst of the night time, it might not happen to be handled using the same attention the day-shift operator had provided.
So, he asked if there seemed to be in any manner to have the controller automatically “fail over” to some second thermocouple.
Multiloop controllers with rack-mount I/O, like Process Automation Controllers (PACs) or high end PLCs, have multiple I/O points and programming that will implement a control strategy for fail-up to another thermocouple as soon as the primary control thermocouple breaks.
WIKA T32 and Honeywell STT350 temperature transmitters with dual inputs for failoverFor some decades, temperature transmitters used at the same time industries have had connections and settings for 2 sensors, and so the output would fail over when the primary sensor (or its wiring) failed caused by a burn-out (open circuit) condition.
WIKA’s T32 transmitter and Honeywell’s STT350 smart temperature transmitter have the dual input fail-over feature (sometimes called redundant sensor operation).
However some industries or shops don’t use either multiloop controllers or temperature transmitters. They connect a thermocouple as well as its extension wire right to an individual loop temperature controller as shown below.
The Honeywell UDC 3200 or 3500 ¼ DIN controllers get it done by alarming on thermocouple break conditions (upscale or downscale) and switching from your primary thermocouple on input #1 to 97dexhpky secondary thermocouple on input #2. It takes approximately two seconds to change over, so it’s fast and disrupts the burner hardly any.
Implementing thermocouple fail-over for UDCs necessitates the Input #2 and digital input options. Here’s the applying note detailing thermocouple fail-over setup for Honeywell UDCs.
The plant agreed with this recommendation to alarm over a failover condition by connecting the controller alarm with an interposing relay into a supervisory system that both annunicates the alarm and sends a message or email on alarm.
A production schedule will not need to be disrupted from the failure of your control thermocouple. An affordable, simple thermocouple fail-over strategy can save your day as soon as the primary thermocouple fails by permitting the furnace load through its cycle.